Apparatus for reproducing quadraphonic sound

ABSTRACT

Two input terminals are separately supplied with a first and a second composite signals, each of which contains at least three signal components having predetermined phase-relationships with one another, a plurality of all-pass phase-shifting networks operative to position the first and second composite signals in phase quadrature with respect to each other are connected to the input terminals, combining networks operative to deliver four output signals, each of which contains a separate dominant signal component, are connected to the all-pass phase-shifting networks. The apparatus further provides separate phase-inverters in the respective all-pass phase-shifting networks to deliver phaseinverted composite signals therefrom, means to sum select portions of the first and second composite signals, means for producing a signal representative of the difference of select portions of the first and second composite signals derived from the phase inverters, and means for comparing the sum and difference signals to produce a signal for controlling the gain of the output signals.

United States Patent [191 Ohsawa June 28, 1974 [21] Appl. No.: 264,030

[30] Foreign Application Priority Data June 21, 1971 Japan 46-44622 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmico Attorney, Agent, or Firm-Lewis H. Eslinger, Esq.; Alvin Sinderbrand, Esq.

[5 7] ABSTRACT Two input terminals are separately supplied with a first and a second composite signals, each of which contains at least three signal components having predetermined phase-relationships with one another, a plurality of all-pass phase-shifting networks operative to position the first and second composite signals in 52 us CL 179 GQ, 79 0 ST, 179/1001 TD phase quadrature with respect to each other are con- 51 lnt.Cl ..H04r 5/00 581 Field of Search.... 179/1 60, 15 BT, 100.4 ST, 179/1001 TD OTHER PUBLICATIONS A Compatable Stereo-Quadraphonic Record System, am O A 51 Septembe .19]

The Compatable Stereo-Quadraphonic SQ Record, Audio Magazine, October, 19 7 1 nected to the input terminals, combining networks operative to deliver four output signals, each'of which contains a separate dominant signal component, are connected to the all-pass phase shifting networks. The

apparatus further provides separate phase-inverters in and second composite signals derived from the phase inverters, and means for comparing the sum and difference signals to produce a signal for controlling the aeiaef lig p s nals, W

5 Claims, 9 Drawing Figures 12 7 l4 i [7F w: w T +5 l w w a;

22 25 III 220 +5 1; KT w KB 245 I, [l e i 2% i i L +R 256 a lil APPARATUS FOR REPRODUCING QUADRAPHONIC SOUND BACKGROUND OF THE INVENTION The invention relates to an apparatus for producing four separate channels of information'signals on a medium having only two independent tracks, and more particularly to improved decoding and reproduction of signals in a four speaker system to give a listener a highly realistic four-channel program.

A matrix four channel stereo system has been heretofore proposed in which four original sound signals (which, for convenience, are identified as L,. L,, R, and R,, for left-front, left-back, right-front and right-back, respectively) are converted into signals of only two media channels by matrix networks, and then they are decoded for reproduction from the two media channels to four signals by matrix networks.

In such systems it is preferred that each original sound signal be reproduced by only one loudspeaker. With such matrix four channel stereo systems, however, in addition to the corresponding original sound signals, another sound signal reproduced through another loudspeaker tends to produce crosstalk. This is especially true when the original sound signals L, and R,, for example, are the same in phase so that during reproduction they are reproduced in the same phase from loudspeakers positioned at the left and right front sides of a listener and are also reproduced in inverse phase from loudspeakers positioned at left and right rear sides of the listener. Conversely, when the original sound signals L,, and R,, are the same in phase, during reproduction they are reproduced in the same phase from the loudspeakers behind the listener at his left and right sides and are also reproduced in inverse phase from the loudspeakers disposed in front of the listener at his left and right sides.

Reproduction of such undesired sounds may be avoided by the provision of a special logic circuit employed in the reproducing apparatus. The logic circuit must be made to discriminate between the characteristics of the original sound signals and particularly as to whether the sound signals L; and R, are in the same phase or the signals L,, and R are in the same phase. The logic circuit must control the gain of a gain control amplifier provided in an independent channel of reproducing networks in accordance with the output thereof. Whether the signals L, and R, are in the same phase or not, or whether the signals L,, and R,, are in the same phase or not is judged by summing and subtracting two composite signals which are provided by converting the original sound signals in a predetermined phase relationship.

In the prior art, the process for subtracting one composite signal from the other composite signal is carried out in such a manner that the one composite signal is directly applied to a mixing circuit and the other composite signal, which is phase-inverted through a phaseinverter, is also supplied to the mixing circuit. With such a process, an additional phase-inverter must be provided with the result that the circuit construction becomes complicated.

SUMMARY OF THE INVENTION Apparatus for reproducing quadraphonic sound signals including a first input terminal to receive a first composite signal containing at least three signal components having predetermined phase-relationships with one another and a second input terminal to receive a second composite signal containing at least three signal components having predetermined phase-relationships with one another, a plurality of all-pass phase-shifting networks connected to the first and second input terminals for positioning the first and second composite signals in phase quadrature with respect to each other, combining networks including gain controllable output amplifiers connected to the all-pass phase-shifting networks for producing four output signals, each output signal containing a separate dominant signal component. The apparatus further includes separate phaseinverters in the respective all-pass phase-shifting networks to deliver phase-inverted composite signals therefrom, means to sum select portions of the first and second composite signals, means for producing a signal representative of the difference of select portions of the first and second composite signals derived from the phase-inverters, and means for comparing the sum and difference signals to produce signals for controlling the gain of the output amplifiers.

Accordingly, a main object of the present invention is to provide an apparatus for reproducing quadraphonic sound in which two composite signals, each having a plurality of signal components with a predetermined phase relationship, are converted into at least four output signals in such a manner that each of the two composite signal components has a dominant component.

Another object of the present invention is to provide an apparatus for reproducing quadraphonic sound in which means are provided for avoiding the fact that when two signal components in two composite signals containing a plurality of signal components are the same in phase, the two signal components are reproduced as cross-talk in the other channels.

A further object of the present invention is to provide an apparatus for reproducing quadraphonic sound in which a circuit of simple construction is provided for preventing cross-talk.

Other objects, features and advantages of the present 7 invention will become more readily apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating an encoder for better understanding of the present invention;

FIGS. 2, 3 and 4, inclusive, are phasor diagrams used for explaining the operation and advantages of the encoder shown in FIG. 1;

FIG. 5 is a schematic diagram illustrating a decoder for decoding signals encoded with the encoder shown in FIG. 1;

FIGS. 6 and 7 are circuit diagrams of all-pass phaseshifting networks, respectively, to be applied to the decoder depicted in FIG. 5; I

FIG. 8 is a phasor diagram illustrating the condition of a reproduced listening field; and

FIG. 9 is a circuit diagram of a preferred embodiment of the invention for reproducing quadraphonic sound.

DESCRIPTION OF THE PREFERRED EMBODIMENT For better understanding the present invention, an

encoder will first be described which produces two composite signals to be supplied to an apparatus for reproducing quadraphonic sound according to the invention.

The encoder illustrated in FIG. 1 has four input terminals 10, 12, 14 and 16 to which the four original sound signals L,, L,,, R,, and R depicted as in-phase signals of equal amplitude, are respectively applied. The total L signal is added in a summing junction 18 to .707 of the R,, signal and the output of the summing junction 18 is supplied to a phase-shifting network 20 which introduces a reference phase-shift I, which is a function of frequency. The full R, signal at terminal 16 is added in a summing network 22 to .707 of the L signal appearing at input terminal 12, and the output is passed through the I -network 24, which also provides the reference phase-shift I. The L,, and R,, signals are also applied to respective networks 26 and 28, each of which provides a phase-shift of I 90. The full signal appearing at the output of network 20 is added in a summing circuit 30 to .707 of the signal appearing at the output of netowrk 26 to produce at its output terminal 32 a composite signal designated L Similarly, the full signal from network 24 is added in summing junction 34 to +.707 of the signal from the network 28. The signal appearing at the output terminal 36 is a composite signal designated R As in the case of most such encoders, the signals L and R may be transmitted by FM multiplex radio or they may be recorded on any two-channel medium such as a two-track tape or stereophonic record for later reproduction.

The significance of the encoder of FIG. 1 will be appreciated from an analysis of the phasor relationship of the L and R composite signals portrayed as phasor groups 38 and 40, respectively. It will be observed that phasor group 38 consists of a component signal L, (which although shown in the same phase relationship as the input signal I has a l -as-a-function-offrequency angle difference between them), a signal component .707R,, in a negative sense with respect to its corresponding input phasor, and a .707L,, signal component which lags the phasor .707R,, by 90 because of the action of the network 26.

Phasor group 40 consists of the original signal component R in the same relative phase position as its corresponding input signal R, but with a P-as-a-functionof-frequency angle difference between them, a signal .707L,, in phase with the R; signal, and a .707R,, signal lagging the .707L,, signal by 90 due to the action of I -network 28. As has been pointed out hereinabove, in the interest of providing better realism of image placement when the record is played in a conventional stereophonic mode over two loudspeakers, it is preferable to arrange for the phasor component .707L,, in the phasor group 40 to lag behind the similarly designated phasor in the phasor group 38, and conversely, to arrange for the phasor component .707R,, in the phasor group 38 to lag behind the corresponding phasor in group 40. Thus, the connections shown in FIG. 1 are preferred.

Referring now to FIG. 2, the effect of panning is to divide the signal (as by means of two coupled attenuators) between two channel inputs. At the midpoint of the panning operation, the signal becomes precisely divided between the front channels L and R or between the back channels L,, or R,,. This condition will now be examined. The phasor groups 38 and 40 from FIG. 1

are repeated here as phasor groups 50 and 52, respectively, and the panned center" signals have been added. The'front center signal, C is divided in the proportion .707C and is in-phase in the phasor groups 50 and 52, appearing as phasors 54 and 56. It is seen, thus far, that the phasor group in FIG. 3 depicts the leftfront channel phasor L; by an arrow 42 and the rightfront channel phasor by an arrow 44 in a relationship which those skilled in the art will recognize as portraying the modulation of a conventional stereophonic record.

Further, it will be noted that the center-back channel C is divided in the proportion .707 in the left back and right back channels, and since these two phasors appear as a .707 fraction, the corresponding fraction of the C signal is 0.5 in phase with the .707L,, phasor and 0.5 in phase with the .707R,, phasors in both phasor groups. With this convention in mind, it is seen that the two phasors in each group add to the larger phasors .707C,, in each of the phasor groups 50 and 52; however, it should also be observed that the phasor .707C,, in phasor group 50 is out-of-phase with the corresponding phasor in group 52. This is an important quality of the encoder of FIG. 1 because now the center-back signal C is of an entirely different character than the center front signal C It will be recognized that the signal C,, having an out-of-phase relationship in the two channels will result in a vertical modulation of the groove of a record disc, which is depicted by the arrow 48 in FIG. 3. Any signal recorded in this manner cannot be reproduced by a monophonic phonograph pick-up, nor by the monophonic section of an FM multiplex transmitting station; consequently, when'using the encoder of FIG. 1 the center-back location should preferably be used for occasional sounds such as reverberation, motion during panning, etc., and not for the placement of an important artist since he would not be heard when the signal is broadcast over AM radio or over monophonic FM radio. Such signals would, however, be fully audible with stereophonic or quadraphonic modes of reproduction, and all other locations of the artist would be transmitted satisfactorily.

Another significant feature of the encoder is illustrated by the phasor groups 56 and 58 in FIG. 4, the former depictingthe situation which results which the phasor groups 50 and 52 of FIG. 2 are added and the latter depicting the situation when the composite signal R (phasor group 52) is subtracted from L (phasor group 50). It will be noted that when L and R are added the phasors L;, L,,, R,, and R, all have an intensity equal to unity, whereas the front center signal C, is augmented by a factor 1.414, which is exactly what happens when a stereophonic record is played over a.

monophonic player. The back center signal C is cancelled, however, because of the aforementioned out-ofphase relationship. When the phasor groups are subtracted, the phasors L,, L,,, R,, and R; again all appear with unity amplitude, but this time the center back signal C,,, is augmented by the factor 1.414 while the center front signal, C is cancelled. The realtionship portrayed by phasor groups 56 and 58 are extremely important since they indicate that if only a center front signal is present (i.e., no center back signal) the phasor group 56 will be greater than group 58, and, conversely, if there is only a center back signal but no center front signal, the phasor group 58 will be the larger. This interesting property is used to advantage to enhance the operation of a decoder, which will now be described, to be utilized with the encoder of FIG. 1.

A decoder to decode such composite signals is illustrated in FIG. 5. In this example, the signals L and R represented by phasor groups 38 and 40, respectively, are applied to respective input terminals 100 and 102, from whence they are applied in parallel to respective pairs of I -networks 104, 106, 108 and 110. In this manner, each of the signals L and R passes without relative phase-shift through networks 104 and 108, respectively, and also passes with a relative phase-shift of 90 through networks 106 and 110. In the phasor groups 112, 114, 116, and 118 portraying the output signals from networks 104, 106, 110 and 108, respectively, the individual phasors, essentially identical with the corresponding phasors in groups 38 and 40, are differentiated with a prime to indicate that they have been subjected to the action of a P-network and thus differ from the input phasors by an angle which is a I'-function of frequency, in addition to the differential angle introduced by the networks.

The outputs of networks 104 and 108 are applied directly to the input terminals of respective gain control amplifiers 152 and 154, the outputs of which are applied to respective loudspeakers 116 and 118. The signals applied to loudspeakers 116 and 118 contain dominant original signals L f and R';, respectively, and the two subdominant contaminating signals .707L,, and .707R,,, respectively. Equal proportions, namely, .707, of the outputs of networks 106 and 108 are summed at a summing junction 120 to produce a composite signal consisting of a dominant signal L',,, which is applied to a gain control amplifier 156 and thence to loudspeaker 124. Equal negative portions, namely, .707, of the outputs of networks 104 and 110 are summed at a second summing network 126 to produce a composite signal composed of a dominant signal, R',,, together with .707R, and .707L After amplification by a fourth gain control amplifier 158 this composite signal is applied to a loudspeaker 130. It will be observed from the phasor groups 132, 134, 136 and 138 which respectively portray the composite signals appearing at loudspeakers 116, 124, 130 and 118, that the predominant phasors at all four loudspeakers are in-phase.

The first to fourth P-networks 104, 106, 108 and 110 shown in FIG. 5 are respectively composed of phaseinverters and phase-shifters. One example of the I -networks, as shown in FIGS. 6 and 7, employs one or more transistors 160 or 162 as phase-inverters and capacitors 164, 166, and 168 and resistors 170 are inserted between the collectors-emitters of the transistors as phase-shifting circuits. The operation of these circuits will be described in greater detail in reference to FIG. 9.

During reproduction of the four channel stereo signal the positional sense with respect to the listeners front and back direction is taken into account. A level difference is provided between the decoded signal corresponding to the front position and the decoded signal corresponding to the back position and this level difference is controlled to position the signals at a predetermined location in the front and back direction. The reproduced sound field in the case where the four channel stereo reproduction is carried out by obtaining the decoded signals is illustrated in FIG. 8. As will be apparent from the figure, the front center signal C,(when L, R',) and the center back signal C (when L,

R',,) are respectively reproduced as cross-talk with the same level but in inverse phases at the left and right in front and also at the back with respect to a listener A, so that the listener feels ill.

Positioning of the signals in a direction at the front and. back of the listener can be derived from the first and second composite signals L and R but also from the sum signal L R and the difference signal L R of the first and second composite signals L and R in the system described above. The sum signal L R and the difference signal L R are shown in FIG. 4 by the phasor groups 56 and 58, respectively. When the L; and R; signals corresponding to the front positions are large, the absolute value IL R I of the sum signal is greater than that of the difference signal IL R I. Conversely, when the signals L,, and R corresponding to the back positions are great, the absolute value lL R I of the sum signal is smaller than theabsolute value IL R of the difference signal. Accordingly, when the condition IL R' I 1L R I is satisfied, the first and fourth decoded signals L and R corresponding to the front positions are relatively enhanced by the circuit of the invention in accordance with the difference, while the second and third decoded signals L and R corresponding to the back position are suppressed correspondingly. When the condition IL R IL R I is satisfied, the second and third decoded signals L and R corresponding to the back positions are relatively enhanced by the circuit of the invention in accordance with the difference, while the first and fourth decoded signals L and R corresponding to the front positions are suppressed.

According to the present invention, the sum and difference signals of the first and second composite signals L and R are provided, their absolute values are then compared to provide a control signal, and the level difference between the first and fourth decoded signals and the level difference between the second and third decoded signals are thereafter controlled with the control signal. In the preferred embodiment of the present invention the first and second composite signals for producing the sum and the difference signals are derived from the phase-inverters of selected ones of the first to fourth I -networks, so that the control circuit construction is simplified. An example of the electric circuits according to th present invention will now be described with reference to FIG. 9 in which the I -networks 121, 222, 232 and 242 shown in FIG. 6 are used. Transistors 213, 223, 233 and 243 serve as phase-inverters for the P-networks 212, 222, 232, and 242. In the FIG. 9, reference numerals 214, 224, 234 and 244 indicate phase-shifters for the I -networks 212, 222, 232 and 242 respectively. The first composite signal L is supplied to the bases of the transistors 213 and 223 of the first and second I -networks 212 and 222, while the second composite signal R is supplied to the bases of the transistors 233 and 243 of the third and fourth I -networks 232 and 242. The first to fourth decoded signals L L,,", R,," and R are respectively obtained by decoding the outputs from the P-networks 212 to 242, and they are then delivered to the loudspeakers through gain control amplifiers 215, 225, 235 and 245. For example, the first composite signal L is derived from the emitter of the transistor 223 which is the phase-inverter of the second I -network 222, and the second composite signals R and -R are derived from the emitter and collector, respectively, of the transistor 243 which is the phase-inverter of the third I -network 242. A summing circuit 220, corresponding to the circuit 120 of FIG. 5, has its inputs connected'to the outputs of the circuits 224 and 234 and its output connected to the output amplifier 225. A summing circuit 226, corresponding to the summing circuit 126 in the circuit of FIG. 5, has its inputs connected to the outputs of the circuits 214 and 244 and its output connected to the output amplifier 245.

The signals L and R are added together through the resistors 250 and 251, connected to the emitters of the transistors 223 and 243, respectively, and the input of an all-wave rectifier circuit 254, to produce the sum signal L R The signals L and R are also added together through resistors 252 and 253, connected to the emitter of transistor 223 and the collector of transistor 243, respectively, and the input of an all-wave rectifier circuit 256, to produce the difference signal L R Thus the sum signal L l R is supplied to an allwave rectifier circuit 254 and the difference signal L R is supplied to an all-wave rectifier circuit 256. The absolute value signals lL l- R I and [L R- l are respectively derived from the all-wave rectifiers 254 and 256 and are then supplied to a subtracter circuit 258 in which both the absolute value signals are compared to produce a control signal. The control signal is supplied directly to the gain control amplifiers 215 and 235 and through a phase-inverter 260 to the other gain control amplifiers 225 and 245.

Accordingly, if an original sound exists at the front center of a listener, the signal C, appears in the sum signal L R as a dominant component, but such a signal component does not appear in the subtracting signal L R After the sum and difference signals are rectified by the all-wave rectifier circuits 254 and 256, they are supplied to the subtracter circuit 258 which delivers at its output terminal a positive control signal representative of those applied thereto. The control signal increases the gains of the gain control amplifiers 215 and 235 but decreases the gains of the gain control amplifiers 225 and 245. Thus, the center signal is reproduced from the loudspeakers placed in front of the listener at his left and right sides, while almost no signal is reproduced from loudspeakers placed behind the listener.

Conversely, if the original sound exists at the back center of the listener, the operation is reversed with the result that the front loudspeakers reproduce almost no signal corresponding to that of the back center.

With the circuit of the present invention described above, upon reproduction of a sound field the center sound in the front and back directioncan be stably positioned, with a circuit of simple construction because in the present invention the first and second composite signals L and R are derived from the phase inverters of the P-networks and based thereupon the sum and difference signals L R and L R respectively, are provided for controlling the levels of the decoded signals without the necessity of providing separate phase inverters.

The phase-shifter shown in FIG. 7 may also be used in the present invention. In this case, the first and second composite signals may be derived from the emitter and collector, respectively, of the first stage transistor 162.

Further, I -networks with transformers as phase inverters may be employed in the present invention.

In the foregoing example, the gains of the gain control amplifiers are controlled by the control signal but the present invention is not to be limited thereto. In fact, it should be noted that the sum and difference signals L R and L R are obtained from the all-pass phase-shifting networks with phase-inverters. Further, the control signal may be used to control the means for deriving the level difference between the front two channel signals and the back two channel signals.

In the foregoing example, the input terminals of the decoder are supplied with two composite signals, respectively, but such a composite signal with all of the signal components, for example, L,, R,, L,, and R,,, in which when the signal components L,, R;, L,, and R are calculated in vector form, the same phase or inverse phase relationship is established between predetermined signal components, may be also used.

It will be apparent that a number of changes and variations can be effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. Apparatus for reproducing quadraphonic sound comprising:

a. first and second input terminals to receive first and second composite signals L and R respectively, containing dominantsignals L and R, in phase with each other, and each including at least two subdominant signals L,, and R having a predetermined phase relationship with each other,

b. first, second, third and fourth all-pass phaseshifting networks, at least one of which includes a phase-inverter, said first and second all-pass phaseshifting networks being connected to said first input terminal, and one of said first and second allpass phase-shifting networks being operative to provide an output signal in response to said first composite signal and in phase quadrature with respect to said second composite signal, and said third and fourth all-pass phase-shifting networks being connected to said second input terminal, and one of said third and fourth all-pass phase-shifting networks being operative to provide an output signal in response to said second composite signal and in phase quadrature with respect to said first composite signal, said phase-inverter being operative to produce a signal corresponding to one of said first and second composite signals but opposite in phase,

c. combining networks connected to said all-pass phase-shifting networks and operative to derive four output signals containing L R L,, and R signals as in-phase, dominant signals, respectively,

works and to the output of said phase-inverter for producing a signal representative of the difference of said first and second composite signals,

f. means for comparing said sum signal and said differe'nce signal and for producing a control signal, and

g. means for varying the signal strength of at least a select pair of said output signals in response to the value of said control signal.

2. Apparatus for reproducing quadraphonic sound as recited in claim 1 in which said all-pass phase-shifting networks each comprise a phase-inverter which includes a plurality of electronic elements having at least first, second and third electrodes, and a phase-shifter which includes separate resistors and capacitors inserted between said second and third electrodes of said electronic elements, said summing means being connected between said second electrode of one of said electronic elements to which said first composite signal is supplied and said second electrode of one of said electronic elements to which said second composite signal is supplied.

3. Apparatus for reproducing quadraphonic sound as recited in claim 2 wherein said difference signal producing means are connected between said second electrode of one of said electronic elements to which said first composite signal is applied and from said third electrode of one of said electronic elements to which said second composite signal is applied.

4. Apparatus for reproducing quadraphonic sound as recited in claim 2 wherein said electronic elements are transistors and said first, second and third electrodes are base, emitter and collector electrodes of said transistors.

5. Apparatus for reproducing quadraphonic sound comprising:

a. first and second input terminals to receive first and second composite signals L and R respectively, containing dominant signals L, and R in phase with each other, and subdominant R,, and L,, signals in quadrature phase relation with each other and with the L, and R dominant signals, the subdominant R,, signal in each of the first and second composite signals being in phase opposition to the subdominant L,, signal in the other of the first and second composite signals,

b. a first all-pass phase-shifting network connected to the first input terminal for shifting the phase of the first composite signal by 90 to produce a third composite signal,

c. a second all-pass phase-shifting network connected to the second input terminal for shifting the phase of the second composite signal by 90 to produce a fourth composite signal;

d. first means for combining the third and second composite signals to produce a fifth composite signal containing a dominant L,, signal component in phase with the dominant signals in said first and second composite signals, and subdominant L, and R, signal components at predetermined relative phases;

e. second means for combining the first and fourth composite signals to produce a sixth composite signal containing a dominant R,, signal component in phase with the dominant signal in said first and second composite and subdominant L and R; signal components at predetermined relative phases, the subdominant L, signal component in each of the fifth and sixth composite signals being in phase opposition to the R; subdominant signal component in the other of the fifth and sixth composite signals;

f. at least a first phase-inverter connected to one of the first and second input terminals for producing a seventh composite signal representative of one of the first and second composite signals respectively, but shifted one-hundred and eight degrees in phase;

g. means connected to the first and second input terminals and to the output of the first phase-inverter for producing a first control signal by taking the difference between the absolute magnitude of the sum of the first and second composite signals and the absolute magnitude of the sum of the seventh composite signal and the other of the first and second composite signals, the polarity of the first control signal being representative of whether the difference in absolute magnitude is greater or less than zero;

h. means responsive to the first, fifth, sixth and second composite signals for separately producing first, second, third and fourth output signals, respectively; and

i. means responsive to the first control signal for increasing the magnitudes of the first and fourth output signals relative to the magnitudes of the second and third output signals when the absolute magnitude of the sum of the first and second composite signals exceed the absolute magnitude of the sum of the seventh composite signal and the other of the first and second composite signals and for decreasing the magnitudes of the first and fourth output signals relative to the magnitudes of the second and third output signals when the opposite is true. 

1. Apparatus for reproducing quadraphonic sound comprising: a. first and second input terminals to receive first and second composite signals LT and RT, respectively, containing dominant signals Lf and Rf in phase with each other, and each including at least two subdominant signals Lb and Rb having a predetermined phase relationship with each other, b. first, second, third and fourth all-pass phase-shifting networks, at least one of which includes a phase-inverter, said first and second all-pass phase-shifting networks being connected to said first input terminal, and one of said first and second all-pass phase-shifting networks being operative to provide an output signal in response to said first composite signal and in phase quadrature with respect to said second composite signal, and said third and fourth all-pass phaseshifting networks being connected to said second input terminal, and one of said third and fourth all-pass phaseshifting networks being operative to provide an output signal in response to said second composite signal and in phase quadrature with respect to said first composite signal, said phase-inverter being operative to produce a signal corresponding to one of said first and second composite signals but opposite in phase, c. combining networks connected to said all-pass phase-shifting networks and operative to derive four output signals containing Lf, Rf, Lb and Rb signals as in-phase, dominant signals, respectively, d. summing means connected to one of said first and second allpass phase-shifting networks and to one of said third and fourth all-pass phase-shifting networks for producing a signal representative of the sum of said first and second composite signals, e. subtracting means connected to one of said first, second, third and fourth all-pass phase-shifting networks and to the output of said phase-inverter for producing a signal representative of the difference of said first and second composite signals, f. means for comparing said sum signal and said difference signal and for producing a control signal, and g. means for varying the signal strength of at least a select pair of said output signals in response to the value of said control signal.
 2. Apparatus for reproducing quadraphonic sound as recited in claim 1 in which said all-pass phase-shifting networks each comprise a phase-inverter which includes a plurality of electronic elements having at least first, second and third electrodes, and a phase-shifter which includes separate resistors and capacitors inserted between said second and third electrodes of said electronic elements, said summing means being connected between said second electrode of one of said electronic elements to which said first composite signal is supplied and said second electrode of one of said electronic elements to which said second composite signal is supplied.
 3. Apparatus fOr reproducing quadraphonic sound as recited in claim 2 wherein said difference signal producing means are connected between said second electrode of one of said electronic elements to which said first composite signal is applied and from said third electrode of one of said electronic elements to which said second composite signal is applied.
 4. Apparatus for reproducing quadraphonic sound as recited in claim 2 wherein said electronic elements are transistors and said first, second and third electrodes are base, emitter and collector electrodes of said transistors.
 5. Apparatus for reproducing quadraphonic sound comprising: a. first and second input terminals to receive first and second composite signals LT and RT, respectively, containing dominant signals Lf and Rf in phase with each other, and subdominant Rb and Lb signals in quadrature phase relation with each other and with the Lf and Rf dominant signals, the subdominant Rb signal in each of the first and second composite signals being in phase opposition to the subdominant Lb signal in the other of the first and second composite signals, b. a first all-pass phase-shifting network connected to the first input terminal for shifting the phase of the first composite signal by 90* to produce a third composite signal, c. a second all-pass phase-shifting network connected to the second input terminal for shifting the phase of the second composite signal by 90* to produce a fourth composite signal; d. first means for combining the third and second composite signals to produce a fifth composite signal containing a dominant Lb signal component in phase with the dominant signals in said first and second composite signals, and subdominant Lf and Rf signal components at predetermined relative phases; e. second means for combining the first and fourth composite signals to produce a sixth composite signal containing a dominant Rb signal component in phase with the dominant signal in said first and second composite and subdominant Lf and Rf signal components at predetermined relative phases, the subdominant Lf signal component in each of the fifth and sixth composite signals being in phase opposition to the Rf subdominant signal component in the other of the fifth and sixth composite signals; f. at least a first phase-inverter connected to one of the first and second input terminals for producing a seventh composite signal representative of one of the first and second composite signals respectively, but shifted one-hundred and eight degrees in phase; g. means connected to the first and second input terminals and to the output of the first phase-inverter for producing a first control signal by taking the difference between the absolute magnitude of the sum of the first and second composite signals and the absolute magnitude of the sum of the seventh composite signal and the other of the first and second composite signals, the polarity of the first control signal being representative of whether the difference in absolute magnitude is greater or less than zero; h. means responsive to the first, fifth, sixth and second composite signals for separately producing first, second, third and fourth output signals, respectively; and i. means responsive to the first control signal for increasing the magnitudes of the first and fourth output signals relative to the magnitudes of the second and third output signals when the absolute magnitude of the sum of the first and second composite signals exceed the absolute magnitude of the sum of the seventh composite signal and the other of the first and second composite signals and for decreasing the magnitudes of the first and fourth output signals relative to the magnitudes of the second and third output signals when the opposite is true. 